Cataclysmic Variables

Introduction to Cataclysmic Variables (CVs)

Cataclysmic
variables (CVs) are binary star systems that have a white dwarf
and a normal star
companion. They are typically small – the
entire binary
system is usually the size of the Earth-Moon system – with an
orbital period
of 1 to 10
hours. The white dwarf is often referred to as the "primary" star, and
the normal star as the "companion" or the "secondary" star. The
companion
star, a star that is "normal," like our Sun, loses material onto the
white dwarf via accretion.
Since the white dwarf is very dense, the gravitational potential energy is
enormous, and some of it is converted into X-rays during the accretion
process. There are probably more than a million of these cataclysmic
variables
in the galaxy, but
only
those near our Sun (several hundreds) have been studied in
X-rays so far. This is because CVs are fairly faint in X-rays; they
are just above the coronal
X-ray sources and far below the X-ray binaries in terms of how
powerful their X-ray emissions are.

A diagram of a
cataclysmic variable, showing the normal star,
the accretion disk, and the white dwarf. The hot spot is
where matter from the normal star meets the accretion disk.

Classical Novae and Dwarf Novae

Optical astronomers discovered CVs based on their outbursts in the
middle of the 19th century. CVs are classified into subclasses
according to the properties of
the outbursts: classical novae and dwarf novae. Classical novae are
seen to erupt once, and the amplitude of the outburst
is the largest among CVs. Classical nova outbursts
are caused by sudden nuclear
fusion of
hydrogen-rich
material on the surface of the white dwarf. Because
white dwarfs are the cinders of stars like the Sun, hydrogen fusion is
possible only when fresh fuel is accreted onto its surface.

Dwarf novae outbursts result from temporary increases in the rate of
accretion onto the white dwarf, caused by the additional material
accreted onto the surface. This material must go through a violent
transition region called the "boundary layer", which lies just above
the surface of the white dwarf. Dwarf novae outbursts are smaller
in amplitude and higher in
frequency than classical novae. The variable star U Geminorum,
or "U Gem," is the prototype of dwarf novae. The brightness in the visible
light of U Gem increases by a hundredfold every 120 days or so, and
returns to the original level after a week or two.

Optical astronomers have also recognized "recurrent novae," which
are eruptive behaviors that fall between the definitions of classical
and dwarf novae,
and "nova-like systems," which are stars that have similar spectra to
other types
of CVs in the visual light, but have not been seen to erupt.

X-ray Emission from CVs

Some X-ray sources
detected by Uhuru turned
out to be CVs; later
on, the Einstein
observatory observed many CVs to be weak X-ray
sources. This is not surprising, as the accreting matter can easily
reach temperatures of 100 million degrees or so near the white dwarf
surface.
Studies of CVs in X-rays therefore reveal the details of the
accretion process near the primary. In the majority of CVs, accretion
proceeds via an accretion
disk. This occurs because the material leaving the secondary has
angular momentum from the binary motion and, therefore, accretion
cannot take place in a straight line. Rather, a disk-like structure is formed in the plane of
the binary orbit (i.e.,
an accretion disk). Friction within the disk heats up the accreting
material, and forces the material to gradually spiral down onto the
white dwarf surface. Scientists think the X-rays in these CVs primarily
come from the boundary layer which lies just above the surface of the
white dwarf. This boundary layer marks where the velocity of the gas
in the disk is forced to match the rotation velocity of the white dwarf.

A diagram of the
companion and accretion disk of a magnetic CV.

However, most of the strongly X-ray emitting CVs turn out to have a
magnetic white dwarf primary (some are known to have a magnetic
field more than a hundred million times stronger than that of
Earth). Since
the accreting material is ionized, this magnetic field can control the
flow. The geometry of accretion is very different in these magnetic
CVs. Accretion disks are truncated or absent, so "column" and "curtain"
are two words used to describe the geometry near the surface. In
these cases, accretion is closer to vertical, along the magnetic field
lines, which results in a stronger shock and stronger X-ray emission
than when the accretion is via a disk. Magnetic CVs have been
discovered mostly through their X-ray emission over the last 30 years.

In some cases, nuclear fusion, rather than accretion, can become the
dominant energy source in a CV. The case of the classical nova outburst
has been mentioned above. In addition, X-ray astronomers have
discovered
a class of objects called the "super-soft sources" (or SSS): the name
derived from the X-ray spectrum of these systems, which is dominated
by
soft (lower energy) X-ray photons, typically below 0.5 keV. Detailed
studies of the spectra of these SSS have revealed that they have
the characteristic of X-rays from the hot (T ~ 200,000 - 800,000K),
high gravity (g ~ 1,000,000 m/s/s) surface of a star. Such high gravity
implies a
white dwarf more massive than
our Sun, which has its own implications.

White dwarf supernovae

Though some matter is ejected during a nova, some may also be
retained, so the accretion/nova cycle can still allow for the dwarf's
mass to increase. This mass gain could eventually result in the dwarf
reaching the Chandrasekhar limit
of 1.4 solar masses. As it approaches that limit, pressure builds
and the internal temperature rises enough for carbon fusion to
begin. The majority of white dwarfs are composed mostly of
carbon, and when this fusion occurs, all the carbon undergoes fusion
instantly. The result is a white dwarf supernova.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.